The synthesis and characterization of a series of dibenzhydryl-based α-diimine Ni(II) complexes bearing a range of electron-donating or -withdrawing groups are described. Polymerization with ethylene is investigated in detail, involving the activator effect, influence of polymerization conditions on catalyst activity, thermal stability, polymer molecular weight and melting point. All of these Ni(II) complexes show great activity (up to 6 × 10⁶ g of PE (mol of Ni)⁻¹·h⁻¹), exceptional thermal stability (stable at up to 100 °C) and generate polyethylene with very high molecular weight (M_n up to 1.6 × 10⁶) and very narrow molecular weight distribution. In the dibromo Ni(II) system, the electronic perturbations exhibit little variation on the ethylene polymerization. In the Ni(acac) system, dramatic ligand electronic effects are observed in terms of catalytic activity and polyethylene molecular weight.

The investigation of the relationship between the structure of the catalyst and the microstructure of the obtained polymer has attracted much attention and broad interest in the field of transition metal-catalyzed olefin polymerization. Cycloalkyls, especially cyclohexyl, are known for their rich conformational change. In this contribution, we described the synthesis and characterization of a series of bulky yet flexible cycloalkyl substituted α-diimine ligands and the corresponding nickel and palladium catalysts. The thermostable nickel complexes in this system show high activity (up to 4.89 × 10⁶ g mol⁻¹ h⁻¹) and can generate highly branched polyethylene (up to 112/1000C) with a high molecular weight (up to 54.4 × 10⁴ g mol⁻¹). The obtained polyethylene displays good elastic properties (SR value up to 77%) characteristic of thermoplastic elastomers. The flexible cycloalkyl substituted palladium complexes exhibit low to moderate catalytic activities (0.6–43.9 × 10⁶ g mol⁻¹ h⁻¹), low to moderate molecular weights (0.93–31.23 × 10⁴ g mol⁻¹), and high branching density (87–122/1000C) for ethylene polymerization, while allowing appreciable comonomer incorporation (1.0–7.7%) for ethylene/MA copolymerization. Most interestingly, compared to the flexible cyclohexyl substituted α-diimine nickel and palladium catalysts, the rigid phenyl substituted catalysts generated polyethylene with much lower branching density in ethylene polymerization.

Previous studies have shown that the introduction of substituents with rigid steric hindrance into α-diimine ligands results in polymers with improved properties via nickel and palladium catalyzed olefin insertion (co)polymerization. In this contribution, a series of α-diimine nickel and palladium complexes containing bulky yet flexible substituents have been synthesized and characterized. The thermostable nickel complexes in this system show high activity and can generate highly branched polyethylene with high molecular weight. Most interestingly, the polymer products display excellent elastic properties (SR value of up to 88%) characteristic of thermoplastic elastomers. In Pd-catalyzed ethylene (co)polymerization, compared with the classical palladium complex Pd0, the presence of flexible substituents in Pd1–Pd3 results in lower activity. However, as the tether length of the flexible substituent increases from Pd1 to Pd3, a clear trend is observed wherein the catalytic activity and polymer molecular weight increase while the polar monomer incorporation and branching density decrease.